Every Tree in the West Has a Climate Secret to Tell

By taking tree ring samples from thousands of trees around the West and determining how old each tree is and how many cycles of wet and dry each has been through, researchers are trying to create the clearest picture of climate in the West over the past several centuries and in turn, hopefully, an equally clear glimpse into the future. Jennifer Pemberton joined scientist Justin DeRose on a field trip to collect tree ring samples and sends this report.

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It's hard to tell how old a juniper is just by looking at it. A tree ring scientist can determine if it's 70 or 700 years old by taking a core sample.

Credit Jennifer Pemberton

First, we have to find some trees. We drive for almost two hours, half of it on a dirt road, and see nothing but sage brush. Justin DeRose chose this spot from aerial photos and doesn’t actually know if the trees are any good here. By any good, he means a couple hundred years old and what he calls “sensitive” to changes in moisture. This sensitivity is expressed in good clear rings -- wider means wetter and thinner means drier.

We crossed the state line just north of Snowville, Utah, and now we’re in Oneida County, Idaho. The road forks up a draw and we drive into what turns into a small canyon. The east slope is a giant landslide of loose rocks. There are some junipers way up high. We grab our equipment and head straight up a scree field. DeRose leads by leaps and bounds, carrying a heavy chainsaw up the slope. When we get to the top, it’s spitting frozen precipitation and the wind is howling. It’s hard to believe trees can live up here, rooted in loose rock, for very long at all.

Justin DeRose uses an increment borer to sample a juniper.

Credit Jennifer Pemberton

DeRose has sampled trees all over the West. In general, Douglas Fir, Limber Pine, and Pinion yield the best climate clues, but in this region, junipers are pretty much the only trees around.

“It appears to be the best species in this region to help us characterize the historical variability in moisture in the hydroclimate," DeRose says.

We don’t need the chainsaw. We have a more refined tool. An increment borer. You don’t need to kill trees to get tree ring samples. These aren’t the big slabs of wood you see in museums. The increment borer is a long thin metal straw. The samples that come out of the tree are thinner than a chopstick. The tree doesn’t even know that part of it is missing.

The increment borer removes thin core samples of the tree. The rings will be counted in the lab to determine the seasons of wet and dry this tree has recorded.

Credit Jennifer Pemberton

As he's drilling into the tree by hand I ask DeRose how deep of a core he needs. "We like to get as many rings as we can," he says. He extracts the core which includes some bark at one end and some clearly defined rings that get tighter towards the other end of the sample -- rings that were formed some time in the 1800s. DeRose estimates this sample is 100 to 150 years old.

The oldest sample we collect today is around 700 years old, by Justin DeRose’s best guess. The dozens of cores collected from these junipers will go back to the lab at Utah State University to be prepped and dated by a machine that reads them by scanning them in like a bar code. They’ll become part of the tree census and all of their climate data will become brushstrokes in the big picture of wet and dry in the West. But more than just a historical document, this picture is a window into the future. A window that water managers and city planners and legislators will be clambering over each other to look through.

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The trees in Utah's forests suck up water like sponges, and leave a record in their growth rings of when there was a lot of water in the region and when there was very little. Researchers are learning to decode the tree ring record and reconstruct what Utah's watersheds have been through over the past millennium. Today on the program, we bring you the story of how Utah's water past can help us plan for Utah's water future.

Special thanks for help with this episode to Western Water Assessment, Wasatch Dendroclimatology Research Group (WADR), and the Utah Climate Center.